1. Field of the Invention
The present invention relates to sensors which can be used in a disk drive for magnetic read back and a manufacturing method thereof.
2. Description of the Background Art
Magnetic sensors in many applications are required to have very high sensitivity. One major example is the use of magnetic read back sensors in disk drives. As the density of recorded information increases with each succeeding product the required sensitivity of the sensor must also increase. Two classes of sensors having very high sensitivity are the magnetic tunnel junction sensor (MTJ) and the charge perpendicular-to-plane sensor (CPP). These sensors both depend on utilizing spin dependent electron movement through a thin nonmagnetic separation layer. On one side of the separation layer is a ferromagnetic layer, called the pinned layer, in which the direction of magnetization is fixed. On the other side of the separation layer is a ferromagnetic layer, called the free layer, in which the direction of the magnetization is free to respond to an applied field. In a disk drive the applied field is from a previously written transition on a disk. In other applications the applied field could come from the position of an external magnet or from the change in location of the sensor relative to a field.
In disk drive applications, it is desirable to achieve maximum sensitivity and linearity. To achieve maximum linearity it is desirable for the magnetization in the free layer in the absence of an external field be substantially parallel with the direction of the recorded track. It is also desirable for the magnetization of the pinned layer to be perpendicular to the magnetization of the free layer in the absence of an applied field. Accordingly it is desired that the magnetization in the pinned layer be substantially perpendicular to the direction of the recorded track.
Another requirement for the free layer is that there be longitudinal magnetic bias stabilization. Imposing a preferred magnetization direction in the free layer along the axis of the free layer parallel to the recording medium and perpendicular to the direction of the track insures good linearity and provides robustness to deleterious effects such as Barkhausen noise.
A common method of providing for the pinning of the pinned layer is to place a layer of antiferromagnetic material (AFM) adjacent to the pinned layer. At some point in the manufacture of the head, the structure is heated above the blocking temperature of the AFM and the device is placed in an external magnetic field which is perpendicular to the eventual direction of the recorded track. The blocking temperature of an AFM material is the temperature above which the material no longer has any exchange coupling strength. The sensor is then cooled in the presence of the field. The applied field will orient the pinned layer in the proper direction and as the AFM cools below the blocking temperature, exchange coupling will maintain the orientation of the magnetization in the pinned layer. For the pinned layer this is the pinning process. This process is also called setting the AFM.
A known method for longitudinal biasing of the free layer is to provide two hard magnets, one on each side of a portion of the free layer. This is referred to as hard biasing. It is generally desirable to electrically insulate the hard bias material from the layers comprising the active sensor. During the manufacture of the sensor, the direction of the magnetization in the hard magnet must be set by placing the sensors in a large magnetic field causing permanent alignment of the direction of magnetization. The requirement of insulating the hard bias magnets is a detractor for this approach.
A preferred method of providing longitudinal bias for the free layer is to use another AFM layer and rely on exchange coupling. The principle difficulty with this approach is that the direction of magnetization in the free layer must be substantially perpendicular to the direction of magnetization in the pinned layer. Thus the steps of heating and subsequent cooling in a field would be appropriate for one of the AFM layers, but not the other. It is known to use two different AFM materials which have two distinctly different blocking temperatures. The AFM layer with the highest blocking temperature is set first. Then the field angle is rotated 900 and the second AFM layer is set at a lower temperature. There is generally one optimum AFM material which would serve for both the pinned layer and longitudinal stabilization of the free layer. However because of the requirement to have AFM materials with different blocking temperatures, the optimum choice of AFM materials has been compromised. What is needed is a sensor structure and a method of manufacturing the sensor which allows for the use of two AFM layers which can be set without compromising other aspects of the sensor.